A gear shift control apparatus for an automatic transmission, which is constructed to include an input shaft, one set of single-pinion type planetary gear, a Simpson type planetary gear train having two sets of single-pinion type planetary gearsets, three clutches, two brakes and an output shaft and which is enabled to achieve six forward speeds and one reverse speed by properly engaging/releasing the three clutches and the two brakes properly that function as gear shift control elements, has been proposed in the prior art in FIG. 2 of JP-A-4-219553.
This gear shift control apparatus constructed of one single-pinion type planetary gear and a Simpson type planetary gear train has the features, as enumerated in the following.    (1) An advantage is taken at the strength because the flow of the torque transmission at the 1st speed for the highest torque of the Simpson type planetary gear train is borne through all members.    (2) An advantage is taken at the gear strength, the gear lifetime, the carrier rigidity and so on, because the Simpson type planetary gear train employs the ring gear input so that the tangential force is reduced to about one half of that of the sun gear input.    (3) For the gear range of an overdrive, there is needed a carrier input to the Simpson type planetary gear train. If the input shaft and the output shaft are coaxially arranged, the input route to the carrier is not satisfied by the single-pinion type planetary gear in which the number of rotary members is limited to three. In order to secure the input route to the carrier, therefore, the input shaft and the output shaft have to be positioned in parallel on the different axis thereby to enlarge the size of the automatic transmission.
In order to solve the aforementioned problem (3), therefore, a gear shift control apparatus using a Ravigneaux type composite planetary gear train (i.e., the composite planetary gear train having sun gears meshing individually with double pinions) in place of the Simpson type planetary gear train has been proposed in FIG. 13, FIG. 14 and FIG. 15 of JP-A-4-219553.
However, the gear shift control apparatus adopting that Ravigneaux type composite planetary gear train has the problems, as enumerated in the following.    (5) The strength is at a disadvantage because the maximum torque (for a 1st speed) of the gear train is borne by the double-pinion type planetary gear on one side of the Ravigneaux type composite planetary gear train.    (6) Another disadvantage resides in the gear strength, the gear lifetime or the carrier rigidity, because the torque augmented by one single-pinion type planetary gear acting as a speed-reducing device is inputted from the sun gear of the Ravigneaux type composite planetary gear train so that the tangential force becomes higher than that of the ring gear input.    (7) The Ravigneaux type composite planetary gear train has to be enlarged to enlarge the automatic transmission, because there are demanded both the securement of the strength (e.g., the gear strength or the gear lifetime) at the 1st speed and the improvement in the carrier rigidity of the Ravigneaux type composite planetary gear train.    (8) Depending upon the gear range, a torque circulation occurs in the Ravigneaux type composite planetary gear train so that the transmission efficiency drops to deteriorate the fuel economy at the gear range where the torque circulation occurs.
In short, the gear shift control apparatus, in which one set of single-pinion type planetary gear and the Ravigneaux type composite planetary gear train, cannot be avoided in the size enlargement of the automatic transmission, because both the aforementioned advantages (1) and (2) of the gear shift control apparatus using one single-pinion type planetary gear and the Simpson type planetary gear train are offset and because the Ravigneaux type composite planetary gear train is enlarged for the aforementioned reason (3).
The present invention has been conceived noting the above-specified problems and has an object to provide a gear shift control apparatus for an automatic transmission, which can make the degree of freedom for selecting a gear ratio higher than that of the case using the Ravigneaux type composite planetary gear train, while achieving a strength advantage (e.g., the gear strength or the gear lifetime) of the gear train, an improvement in the fuel economy, the coaxial positioning of the input portion and the output portion and the size reduction of the automatic transmission altogether.